The invention relates generally to the field of optical measurements and measuring systems, and more particularly to a method and system for optical spectrum analysis that utilizes optical heterodyne detection.
Dense wavelength division multiplexing (DWDM) requires optical spectrum analyzers (OSAs) that have higher spectral resolution than is typically available with current OSAs. For example, grating based OSAs and autocorrelation based OSAs encounter mechanical constraints, such as constraints on beam size and the scanning of optical path lengths, which limit the degree of resolution that can be obtained. As an alternative to grating based and autocorrelation based OSAs, optical heterodyne detection systems can be utilized to monitor DWDM systems.
Optical heterodyne detection systems are utilized for optical spectrum analysis of an input optical signal. FIG. 1 is a depiction of a prior art heterodyne-based detection system that includes an optical coupler 110 that combines an input signal 102 from an input fiber 104 with a swept local oscillator signal 106 from a local oscillator source 105 via local oscillator fiber 108. The combined optical signal travels on an output fiber 118 and is detected by a receiver 112. The square law detection results in mixing of the two combined waves and produces a heterodyne beat signal at a frequency that is equal to the frequency difference between the combined waves. The receiver converts optical radiation from the combined optical signal into an electrical signal. The electrical signal is processed by a signal processor 116 to determine a characteristic of the input signal, such as frequency, wavelength, or amplitude. In order to prevent fading of the heterodyne beat signal, it is important that the polarization states of the input signal and the swept local oscillator signal are matched. One technique for matching the polarization states of the input signal and the swept local oscillator signal involves adjusting the polarization state of the swept local oscillator signal with a polarization controller 120 to track changes in the polarization state of the input signal. A disadvantage of the polarization matching technique is that a polarization tracking system is required.
A polarization diversity receiver can be incorporated into a heterodyne-based OSA to provide polarization independent signal detection. FIG. 2 is a depiction of a heterodyne-based OSA that incorporates a polarization diversity receiver. Throughout the specification, similar elements are identified by similar reference numerals. The heterodyne-based OSA includes a polarization controller 220 on the local oscillator fiber 208, an optical coupler 210, a polarizing beam splitter 224, two receivers 212 and 214, and a processor 216. The polarizing beam splitter splits the combined optical signal into two orthogonally polarized beams that are separately detected by the respective receivers. The power of the local oscillator signal 206 is split equally between the receivers 212 and 214 by adjusting the polarization state of the local oscillator signal. The orthogonally polarized beams that are detected by the two receivers include an intensity noise component and the heterodyne beat signal, as is known in the field of optical heterodyne detection. The strength of the heterodyne beat signal is recovered by squaring and adding the individual signals from both detectors. Although this technique works well, in order to obtain the desired dynamic range in the heterodyne-based OSA, noise subtraction is required in addition to polarization diversity. In order to accomplish noise subtraction, a balanced receiver with four detectors may be utilized, however this increases the complexity and cost of the OSA. Furthermore, the simultaneous implementation of polarization diversity and noise subtraction is possible only if the birefringence within the output leads of the coupler is nearly identical.
In view of the prior art limitations, what is needed is an optical heterodyne detection system that provides polarization independence and intensity noise suppression.
An embodiment of the invention involves an optical spectrum analyzer in which a swept local oscillator signal is depolarized before the swept local oscillator signal is combined with an input signal. Combining the depolarized swept local oscillator signal with the input signal generates a heterodyne beat signal that is used to determine the optical frequency or wavelength of the input signal. The swept local oscillator signal is depolarized in order to prevent fading of the heterodyne beat signal that is caused by changes in the polarization state of the input signal. In an embodiment, the swept local oscillator signal is depolarized such that the average polarization state of the swept local oscillator signal contains no preferential polarization state. That is, the center of gravity of all polarization states experienced by the local oscillator signal is at the center of the Poincarxc3xa9 sphere. Having an average polarization state at the center of the Poincarxc3xa9 sphere ensures that half of the power of the swept local oscillator signal always has the same polarization state as the input signal, thereby keeping the power of the heterodyne beat signal constant regardless of the input signal""s polarization state.
An embodiment of a method for optical spectrum analysis that utilizes optical heterodyne detection involves providing an input signal, providing a swept local oscillator signal, depolarizing the swept local oscillator signal, combining the input signal with the depolarized swept local oscillator signal to create a combined optical signal, detecting the combined optical signal, and generating an output signal that is indicative of an optical parameter of the input signal.
In an embodiment of the method, the step of depolarizing the swept local oscillator signal includes a step of phase modulating the swept local oscillator signal. In an further embodiment, orthogonal polarization components of the swept local oscillator signal are modulated independently, resulting in polarization modulation. In a further embodiment, the polarization modulation creates a pseudo-depolarized swept local oscillator signal. In an further embodiment, the average polarization state of the pseudo-depolarized swept local oscillator signal is at the center of the Poincarxc3xa9 sphere. In an alternative embodiment, the average polarization state of said pseudo-depolarized swept local oscillator signal is within 20 percent of the Poincarxc3xa9 sphere radius from the center of the Poincarxc3xa9 sphere.
In another embodiment, the step of depolarizing includes birefringence modulation. In a further embodiment, the swept local oscillator signal is linearly polarized at 45 degrees with respect to the birefringent axes of a depolarizing component before being depolarized.
In another embodiment, step of depolarizing includes steps of splitting the swept local oscillator signal into at least two portions, delaying one of the at least two portions, and transforming the polarization state of one of the at least two portions such that the at least two portions have orthogonal polarization states.
An embodiment of a system for optical spectrum analysis includes a depolarizer, an optical coupler, and a heterodyne receiver. The depolarizer has an input to receive a swept local oscillator signal and an output for outputting a depolarized swept local oscillator signal. The optical coupler has a first input and a second input, the first input receiving an input signal, the second input being optically connected to the depolarizer to receive the depolarized swept local oscillator signal. The optical coupler also has an output for outputting a combined optical signal that includes the input signal and the depolarized swept local oscillator signal. The heterodyne receiver has an input for receiving the combined optical signal from the optical coupler and an output for outputting an electrical signal representative of the combined optical signal.
In an embodiment, the optical spectrum analyzer includes a processor for receiving the electrical signal from the optical receiver and generating an output signal that is indicative of an optical parameter of the input signal, wherein the processor monitors a heterodyne beat signal that is a component of the combined optical signal.
An embodiment further includes a tunable laser optically connected to the depolarizer for generating the swept local oscillator signal. In an embodiment, the depolarizer is a phase modulator. In an embodiment, the phase modulator is a birefringent phase modulator. In an embodiment, the birefringent phase modulator is a Ti-indiffused LiNbO3 phase modulator. In an embodiment, the birefringent phase modulator outputs the depolarized swept local oscillator signal with an average polarization state that is at the center of the Poincarxc3xa9 sphere. In an embodiment, the birefringent phase modulator outputs the depolarized swept local oscillator signal with an average polarization state that is within 20 percent of the Poincarxc3xa9 sphere radius from the center of the Poincarxc3xa9 sphere.
An embodiment of the depolarizer includes a splitter for splitting the swept local oscillator signal into at least two portions, a delay for delaying one of the at least two portions, and a component for transforming the polarization state of one of the at least two portions such that the at least two portions have orthogonal polarization states.
In an embodiment, the depolarizer includes multiple depolarizers. In a further embodiment, the multiple depolarizers are cascaded.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.